This application claims Paris Convention priority of DE 102 27 877.6 filed Jun. 22, 2002 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a magnet assembly for generating a magnetic field in the direction of a z axis in a working volume disposed on the z axis about z=0 with an actively shielded superconducting magnet coil system and at least one current path which is superconductingly closed in the operating state, wherein the actively shielded superconducting magnet coil system comprises a radially inner and a radially outer partial coil system which are disposed coaxially to each other and whose magnetic dipole moments have opposite signs in the operating state which differ by an amount of xcex94m with |xcex94m| less than 2.5% of the magnetic dipole moment magnitude of the radially inner partial coil system, wherein the magnetic field of the magnet assembly can be expanded along the z axis about z=0 in a polynomial in z having the coefficients Hn and wherein the magnet assembly has an external device for charging the superconductingly closed current paths with an operating current in the operating state and wherein the overall contribution of the superconductingly closed current paths to the magnetic field of the magnet assembly in the working volume of the magnet assembly in the operating state is smaller than 5% of the field contribution of the actively shielded superconducting magnet coil system.
A magnet assembly of this type comprising an actively shielded magnet coil system and at least one additional superconductingly closed current path is disclosed in the patent document U.S. Pat. No. 6,265,960. In this magnet assembly, an additional, superconductingly closed current path acts as superconducting shim device to improve the field homogeneity in the working volume of the magnet assembly.
Patent document WO 00/52490 discloses a further magnet assembly comprising an actively shielded magnet coil system and at least one additional superconductingly closed current path. This magnet assembly comprises an additional superconductingly closed current path for compensating external electromagnetic disturbances, for compensating a field drift caused by the magnet coil system itself, or for fine adjustment of the magnetic field strength in the working volume.
Superconducting magnets have various fields of application which include high-field applications, e.g. for magnetic resonance methods. Such high-field magnets also typically generate a large fringe field. This fringe field can represent a danger for the surroundings of the magnet. This problem can be solved when the magnet comprises an active shielding, i.e. an additional superconducting coil which is connected in series with the main coil of the magnet but which generates a field of opposite polarity.
In particular magnets with highly efficient fringe field shielding pose the problem that deviations from the design specifications for the magnet coils may cause considerable changes in the fringe field generated by the magnet assembly such that required fringe field specifications are not met. Small deviations from the design specifications due to production tolerances are unavoidable. For example, the wire diameters may have tolerances of up to one percent. Such small deviations can dramatically deteriorate the fringe field values since large field contributions with different signs are mutually superposed for compensating the fringe field outside of the magnet assembly. At a location where a fringe field of 0.5 mT should result, the mutually compensating amounts of main coil and fringe field shielding are e.g. in an order of magnitude of 100 mT. A deviation of one of these two field contributions from its desired value by approximately 1% caused by the production inaccuracies in the coil system produces a deviation of the fringe field strength from the desired value of approximately 1 mT at the location of the 0.5 mT contour surface. The required fringe field limit at this location could thereby be exceeded; in this case by multiple factors.
It is the underlying purpose of the present invention to improve a conventional magnet assembly such that its fringe field boundary values are maintained even when individual parameters of the coil arrangement differ from the desired values in consequence of production inaccuracies. In particular, fringe field effects from deviations in the winding data from their desired values and due to geometrical deviations in the coil formers carrying the individual coil systems from their desired geometry, shall be compensated for.
This object is achieved in accordance with the invention in that the magnet assembly comprises at least one additional superconducting current path, which can be charged independently of the actively shielded magnet coil system. To permit improvement of the fringe field of the magnet assembly by the additional current paths without negative side effects on the function of the magnet assembly, the dimensioning of these current paths must meet various requirements. The field contribution generated by the additional current paths in the working volume of the magnet assembly should not exceed 5% of the field contribution value of the actively shielded superconducting magnet coil system. Moreover, it should be ensured that the currents flowing in the additional current paths do not substantially deteriorate the homogeneity of the magnetic field of the magnet assembly in the working volume. Since the homogeneity of the magnet assembly is mainly determined by the coefficient of second order in the polynomial expansion of the magnetic field about the center, the contribution of the additional current paths to the coefficient of second order of the overall field of the magnet assembly should be small compared with the corresponding coefficient of the actively shielded superconducting magnet coil system and/or not exceed an absolute maximum value of 0.25T/m2. These conditions are also at least partially met by additional superconductingly closed current paths in accordance with the cited prior art.
In one inventive embodiment of the additional, superconductingly closed current path, one ensures that the area enclosed by its windings is large enough that the currents flowing therein generate a sufficiently large fringe field contribution, in particular a sufficiently large magnetic dipole moment. In this manner, suitable selection of the currents flowing in the individual additional current paths in the operating state of the configuration can at least partly compensate for a deviation of the magnetic dipole moment of the actively shielded magnet coil system from its desired value caused by production tolerances via the dipole moment of the currents flowing in the additional superconductingly short-circuited current paths without substantially deteriorating other properties of the magnetic field of the overall configuration by field contributions of the additional superconductingly short-circuited current path or paths.
The inventive configuration is advantageous in that even for very well actively shielded superconducting magnet systems whose fringe field reacts excessively to deviations in the design parameters of the magnet coil arrangement from their desired values, the theoretically achievable fringe field limits can be kept without having to take expensive and demanding measures during production to prevent such deviations from the design specifications. This permits more economical manufacture requiring no additional measures for precise production tolerances in the winding data in accordance with design specifications, such that wires and coil formers having precise tolerances are not required. One particular advantage of the inventive configuration is that it can prevent excessive fringe field values caused by deviations of various design parameters of the actively shielded magnet coil system from their desired values without having to calculate and construct a special configuration for each individual parameter.
In a particularly advantageous inventive magnet assembly, the magnetic dipole moments of the partial coil systems of the actively shielded magnet coil system differ by less than 1% of the magnetic dipole moment of the radially inner partial coil system. In this case, the production tolerance related deviations of the dipole moments of the two partial coil systems of the actively shielded magnet coil system are particularly large relative to the overall dipole moment of the magnet coil system. This produces large deviations in the overall dipole moment of the magnet coil system from its theoretical value even for small deviations in the design parameters of the partial coil systems from their desired values. It is therefore advantageous, in particular for such actively shielded magnet coil systems, to provide an inventive device in the form of additional superconductingly short-circuited current paths for compensating deviations of the fringe field of the magnet coil system from its desired value.
One embodiment of the inventive magnet assembly is also particularly advantageous with which the magnet assembly is part of an apparatus for high-resolution magnetic resonance spectroscopy. The radially inner partial coil system of such magnet assemblies generally has a very large dipole moment due to the high field strengths required for such apparatus, and therefore the use of actively shielded magnet systems is particularly advantageous. The inventive configuration ensures that production deviations of the magnet coil system from its design specifications do not result in excessive fringe fields as would be particularly undesirable in this case due to the usual large dipole moments.
One embodiment of the inventive magnet assembly is particularly preferred in which the magnet assembly comprises a further superconducting coil system which serves as superconducting Z2 shim for the magnet assembly and whose contribution to the magnetic field of the magnet assembly largely eliminates the coefficient of second order of the polynomial expansion of the magnetic field of the magnet assembly in the operating state. Deterioration of the field homogeneity of the magnet assembly which may be caused by somewhat inhomogeneous field contributions from the additional superconductingly closed current paths can at least be partially compensated for by providing the magnet assembly with an additional Z2 shim. In this manner, the additional current paths only effect a change in the external fringe field but do not cause deterioration in the field homogeneity at the center of the magnet assembly.
In one particularly preferred embodiment of the inventive magnet assembly, at least one of the additional superconductingly short-circuited current paths is inductively decoupled from the overall magnet coil system. An inductive coupling between the magnet coil system and a superconductingly short-circuited current path would require its switch to be regularly opened during charging to prevent undesired inductive charging of that current path. In the preferred embodiment configuration, no current change is induced in the actively shielded magnet coil system even during charging of the additional current paths to their respective operating current and an undesired field shift in the working volume is thereby avoided.
In a further advantageous embodiment of the inventive magnet assembly, at least one of the additional superconductingly short-circuited current paths is part of a device, which provides the magnet assembly with additional functionality. In particular, that current path serves for compensating external magnetic field fluctuations. This embodiment is advantageous since this double functionality makes the overall magnet assembly more compact.
In a further particularly preferred embodiment of the inventive magnet assembly, at least one of the additional superconductingly short-circuited current paths is thermally decoupled from the actively shielded magnet coil system. In the event of a quench in the magnet coil system, the amount of heat transferred to this additional current path is thereby, at least initially, too small to trigger a quench in the additional current path. This is important since the current induced in the additional, still superconducting, current path due to the change in the magnetic flux during the initial phase of the quench of the magnet coil system, is needed in order to keep the fringe field of the magnet assembly small, even during a quench.
One embodiment of the inventive magnet assembly is particularly preferred wherein at least one of the additional superconductingly closed current paths has magnet coils of a radius which is at least 90% of the radius of the outermost coils of the actively shielded superconducting magnet coil system. Since this current path surrounds a large surface, it generates a considerable magnetic dipole moment without requiring a large current. This is advantageous since the maximum current, which flows in this current path, can be easily kept below the critical current above which the superconductivity would break down.
One further advantageous embodiment of the inventive magnet assembly is characterized in that at least one of the additional superconductingly closed current paths comprises coils which are wound on a coil former which also serves as a carrier for coils of the outer partial coil system of the actively shielded superconducting magnet coil system. This additional current path thereby surrounds a large surface and therefore requires only a small amount of current to generate a magnetic dipole moment, which has sufficient influence on the fringe field. The production of the magnet assembly is less expensive, since no additional coil former is required for the additional current path.
One particularly preferred embodiment of the inventive magnet assembly is characterized in that the overall contribution of the additional superconductingly closed current paths to the magnetic field of the magnet assembly in the working volume of the magnet assembly in the operating state is negligibly small. This advantageously permits simplified installation of the system to obtain the desired field in the working volume and an optimum fringe field outside of the magnet assembly. Setting the currents in the additional current paths does not lead to considerable change in the field magnitude within the working volume.
In one further advantageous embodiment of the inventive magnet assembly, at least one of the additional superconductingly closed current paths generates an asymmetrical magnetic field in the operating state relative to a plane perpendicular to the z axis and intersecting same at z=0. This embodiment permits correction of deviations in the fringe field of the magnet assembly from its desired behavior, which are asymmetrical relative to this plane. Fringe field distortions of this type can be caused e.g. by superconducting shim coils in the magnet assembly if the shim coils produce a field contribution which can be expanded along the z-axis about z=0 in a polynomial in z with uneven coefficients Hn.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used in accordance with the invention either individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and is explained in more detail with reference to embodiments.